Preparation of metallocene containing cationic polymers for anion exchange applications

ABSTRACT

Chemically inert, mechanically tough, cationic metallo-polyelectrolytes designed as durable anion-exchange membranes (AEMs) via ring-opening metathesis polymerization (ROMP) of cobaltocenium-containing cyclooctene with triazole as the only linker group, followed by backbone hydrogenation to provide a new class of AEMs with a polyethylene-like framework and alkaline-stable cobaltocenium cation for ion transport, which exhibit excellent thermal, chemical and mechanical stability, as well as high ion conductivity.

BACKGROUND OF THE INVENTION 1) Field of the Invention

The current disclosure provides chemically inert, mechanically tough,cationic metallo-polyelectrolytes designed as durable anion-exchangemembranes (AEMs) via ring-opening metathesis polymerization (ROMP) ofcobaltocenium-containing cyclooctene with triazole as the only linkergroup, followed by backbone hydrogenation, and a new class of AEMs witha polyethylene-like framework and alkaline-stable cobaltocenium cationfor ion transport, which exhibit excellent thermal, chemical andmechanical stability, as well as high ion conductivity.

2) Description of Related Art

Anion exchange membrane fuel cells (AEMFCs) are currently garneringrenewed attention. In an AEMFC, an anion exchange membrane (AEM)conducts hydroxide (or carbonate) anions (as opposed to protons) duringcurrent flow, which results in several advantages. First, the oxygenreduction reaction (ORR) is much more facile in alkaline environmentsthan in acidic environments. This could potentially facilitate the useof less expensive non-PGM catalysts with high stability in alkalineenvironments. Second, the electro-oxidation kinetics for many liquidfuels (including non-conventional choices of importance to the military,such as sodium borohydride) are enhanced in an alkaline environment.Third, the electroosmotic drag associated with ion transport opposes thecrossover of liquid fuel in AEMFCs, thereby permitting the use of moreconcentrated liquid fuels. This is an advantage for portableapplications. Fourth, the flexibility in terms of fuel and ORR catalystchoice also expands the parameter space for the discovery of highlyselective catalysts that are tolerant to crossover fuel. These potentialadvantages make AEMFCs an attractive proposition.

AEMs have attracted much interest in recent decades because AEM fuelcells use less expensive metallic catalyst and provide higherefficiency. Various polymers are used for AEMs such as poly(ethyleneoxide) (PEO), poly(phenylene oxide) (PPO), poly(benzimidazole) (PBI) andpoly(phenylene) (PP) with quaternary ammonium or imidazolium cations.Many of related designs are limited by rigidity of aromatic backbonesand instability of these cations in basic conditions.

Polyelectrolytes have vast applications ranging from coating tobiomedical materials. Among them, cationic polyelectrolytes are widelypursued as AEMs in alkaline fuel cells for converting fuels intoelectricity, which can be operated below 100° C. and thus are moreattractive as energy conversion devices for electric vehicles andpowering portable devices.

Commonly, AEMs are copolymers containing cations that are covalentlylinked to hydrophobic polymer backbones. The most popular approach isthe use of quaternary ammonium cation (QAC)-containing polymers.However, QACs are prone to degradation through Hofmann elimination (E2)or nucleophilic substitution (S_(N)2) under highly basic conditions.Over the past few years, several strategies have been adopted to improvethe alkaline stability of AEM backbones: 1) protect ammonium cationsusing steric hindrance, conformational restrictions or inductiveeffects; 2) explore alternative stable cations such as benzyl quaternaryguanidinium, imidazolium, and phosphonium to slow down degradation.However, these methods have either sacrificed ion conductivity or stillfaced challenges on achieving long-term stability.

Of equal importance, other components of polyelectrolytes as AEMs arealso required to be stable for withstanding harsh chemical environments.A degradation or deterioration of chemical compositions ofpolyelectrolytes would have a catastrophic impact on membraneperformance. It is essential to design novel cationic polyelectrolyteswith high cation stability and superior chemical durability of thepolymer framework.

Most current anion exchange materials suffer from poor mechanical andbase stability. Accordingly, it is an object of the present disclosureto provide new synthesis methods for making metallocene containingpolymers, as well as to improve the physical properties of thesepolymers. Compared to PEO, PPO, PBI and PP based polymers, the vinylmonomers of the current disclosure allow for simpler tuning of thepolymer properties of metallocene-containing polymers by changing themonomer structures.

SUMMARY OF THE INVENTION

The above objectives are accomplished according to the present inventionby providing in one embodiment an anion-exchange membrane comprising acationic metallo-polyectrolyte that includes a polymer backbone with atriazole group linker between the polymer backbone and cobaltocenium andthe anion-exchange membrane has a polyethylene-like framework andalkaline-stable cobaltocenium for ion transport. Further, the membraneis flexible. Still further, the polymer backbone comprisesnonhydrolysable hydrocarbon. Even further, the anion-exchange membraneis not soluble in polar aprotic solvents. Further still, the polymerbackbone is hydrophobic. Yet further, the anion-exchange membraneincludes hydrophilic side-chains to facilitate ion transport. Moreover,ion transport is facilitated via connected ionic channels. Yet again,the anion exchange membrane exhibits increased conductivity astemperature increases.

In an alternative embodiment, a method is provided for preparing ananion-exchange membrane. The method includes performing a catalyzedclick reaction to attach cobaltocenium onto a cyclic vinyl monomer,performing ring opening metathesis polymerization of the cyclic monomer,wherein the cyclic vinyl monomer comprises cobaltocenium with triazolelinker groups, performing backbone hydrogenation of a resultingcopolymer of the ring opening metathesis polymerization to form ahydrogenated copolymer, and producing a polyethylene-like framework andalkaline-stable cobaltocenium cation for ion transport. Further, themethod includes eliminating hydrolysable functionalities from the anionexchange membrane. Still further, the method includes subjecting thehydrogenated copolymer to ion-exchange from hexafluorophosphate (PF₆—)to chloride (Cl—) and then to hydroxide (OH—). Further yet, the cyclicvinyl monomer comprises cyclooctene. Even further, the anion-exchangemembrane is not soluble in polar aprotic solvents. Still further yet,the anion-exchange membrane is flexible.

In a further alternative embodiment, an anion-exchange membrane formedvia a reaction scheme as shown in FIG. 9A.

BRIEF DESCRIPTION OF THE DRAWINGS

The construction designed to carry out the invention will hereinafter bedescribed, together with other features thereof. The invention will bemore readily understood from a reading of the following specificationand by reference to the accompanying drawings forming a part thereof,wherein an example of the invention is shown and wherein:

FIG. 1 shows examples of styrene, cyclooctene, norbornene andcyclic-type of cationic metallocene-containing cationic vinyl monomersand structures of multi-substituted metallocenium cations can beapplied.

FIG. 2 shows examples of preparation of a mono- and multi-substitutedcobaltocenium-containing cyclooctene monomer and copolymers.

FIG. 3 shows examples of improving the properties of cobaltoceniumcopolymers by hydrogenation (a) and crosslinking (b).

FIG. 4 demonstrates the mechanical properties of hydrogenatedcobaltocenium-containing copolymer (a) and crosslinked copolymer (b).

FIG. 5 shows conductivity verses temperature for hydrogenatedcobaltocenium-containing copolymer (a) and crosslinked copolymer (b).

FIG. 6 illustrates conductivity verses immersed time in 80° C. 1M KOHsolution for hydrogenated cobaltocenium-containing copolymer (a) andcrosslinked copolymer (b).

FIG. 7 shows mechanical properties of anion-exchange membranes based onmulti-substituted cobaltocenium: normal cobaltocenium (blue),dimethyl-cobaltocenium (orange) and octamethyl cobaltocenium (green).

FIG. 8 shows thermal stability of anion-exchange membranes based onmulti-substituted cobaltocenium: normal cobaltocenium (blue),dimethyl-cobaltocenium (orange) and octamethyl cobaltocenium (green).

FIG. 9A shows Scheme 1, which shows synthesis of cobaltocenium monomer2, copolymer 3 and hydrogenated polymer 4 as AEMs.

FIG. 9B shows images of transparent and flexible cobaltocenium AEMs.

FIG. 9C shows proposed structures of cobaltocenium containing AEMS in ahydroxide solution.

FIG. 10 shows ¹H NMR spectra of cobaltocenium monomer 2 (top) in CDCl₃,copolymer 3 (middle) and hydrogenated polymer 4 (bottom) in dimethylsulfoxide-d₆.

FIG. 11 shows Table 1: Properties of cobaltocenicum AEMs.

FIG. 12 shows cobaltocenium AEMS: (a) TGA and DTG curves; (b) SAXSprofiles; (c) hydroxide conductivity of membranes as function oftemperature in fully hydrated condition; and (3) room temperatureconductivity as a function of immersion time of membranes in 1 M NaOHsolution at 80° C.

It will be understood by those skilled in the art that one or moreaspects of this invention can meet certain objectives, while one or moreother aspects can meet certain other objectives. Each objective may notapply equally, in all its respects, to every aspect of this invention.As such, the preceding objects can be viewed in the alternative withrespect to any one aspect of this invention. These and other objects andfeatures of the invention will become more fully apparent when thefollowing detailed description is read in conjunction with theaccompanying figures and examples. However, it is to be understood thatboth the foregoing summary of the invention and the following detaileddescription are of a preferred embodiment and not restrictive of theinvention or other alternate embodiments of the invention. Inparticular, while the invention is described herein with reference to anumber of specific embodiments, it will be appreciated that thedescription is illustrative of the invention and is not constructed aslimiting of the invention. Various modifications and applications mayoccur to those who are skilled in the art, without departing from thespirit and the scope of the invention, as described by the appendedclaims Likewise, other objects, features, benefits and advantages of thepresent invention will be apparent from this summary and certainembodiments described below, and will be readily apparent to thoseskilled in the art. Such objects, features, benefits and advantages willbe apparent from the above in conjunction with the accompanyingexamples, data, figures and all reasonable inferences to be drawntherefrom, alone or with consideration of the references incorporatedherein.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

With reference to the drawings, the invention will now be described inmore detail. Unless defined otherwise, all technical and scientificterms used herein have the same meaning as commonly understood to one ofordinary skill in the art to which the presently disclosed subjectmatter belongs. Although any methods, devices, and materials similar orequivalent to those described herein can be used in the practice ortesting of the presently disclosed subject matter, representativemethods, devices, and materials are herein described.

Unless specifically stated, terms and phrases used in this document, andvariations thereof, unless otherwise expressly stated, should beconstrued as open ended as opposed to limiting. Likewise, a group ofitems linked with the conjunction “and” should not be read as requiringthat each and every one of those items be present in the grouping, butrather should be read as “and/or” unless expressly stated otherwise.Similarly, a group of items linked with the conjunction “or” should notbe read as requiring mutual exclusivity among that group, but rathershould also be read as “and/or” unless expressly stated otherwise.

Furthermore, although items, elements or components of the disclosuremay be described or claimed in the singular, the plural is contemplatedto be within the scope thereof unless limitation to the singular isexplicitly stated. The presence of broadening words and phrases such as“one or more,” “at least,” “but not limited to” or other like phrases insome instances shall not be read to mean that the narrower case isintended or required in instances where such broadening phrases may beabsent.

In various aspects, the present disclosure provides the preparation ofmetallocene-containing cationic monomers and polymers prepared fromthese monomers. Vinyl monomers prepared with metallocene cations resultin polymer molecules with these cations as functional groups.

The metallocene moiety can be mono-substituted or multi-substitutedcobaltocenium, rhodocenium and their derivatives. Metallocene-containingvinyl monomers include styrene, cyclooctene, norbornene and cyclicmonomers. Diverse linkers can be placed between the vinyl groups andmetallocene-containing functional moieties. In a preferred embodiment,the substituted cobaltocenium cation may be employed due to itsextraordinary stability toward highly basic conditions.

Polymer compositions of the current disclosure include homopolymers,random copolymers, and block copolymers (linear copolymer, starcopolymers, bottle-brush copolymers, etc., as known to those of skill inthe art). These polymers have metallocene moiety at either side-chain orbackbones.

The present disclosure provides an approach to the synthesis ofmetallocene-containing polymers by using different polymerizationmethods, and such polymers have applications in many fields, such asanion-exchange, gas separation, electrodialysis, redox flow batteries,fuel cells, etc.

More particularly, methods are provided for preparingmetallocene-containing cationic vinyl monomers and polymers, wherein:(i) said metallocene-containing cationic vinyl monomers include styrenemonomers, cyclooctene monomers, norbornene monomers and cyclic monomersthat can be polymerized; (ii) said metallocene moiety that can bemono-substituted or multi-substituted cobaltocenium, rhodocenium andtheir derivatives; (iii) said homopolymers, random copolymers, blockcopolymers (linear copolymers, star copolymers, bottle-brush copolymers,etc., as known to those of skill in the art) that derive frompolymerization of metallocene-containing cationic vinyl monomers; and(v) said homopolymers, random copolymers, block copolymers (linearcopolymers, star copolymers, bottle-brush copolymers, etc., as known tothose of skill in the art) that have applications as anion exchangematerials for fuel cell applications.

In various embodiments, metallocene cation is attached as the sidegroups or in the backbones of polymers. Sidechain cationic metallocenecontaining polymers include styrenes, cyclooctenes and norbornenes. Mainchain cationic metallocene containing polymers are generally made fromcyclic metallocene monomers via ring opening polymerization (ROP). Themechanical properties and alkaline stabilities of these polymers couldbe further improved by crosslinking or hydrogenation. The anionconductivity is high. The disclosure is further directed to methods ofits use. In on embodiment, a hydrogenated polymer was produced with 50%cobaltocenium cation loading and reached 74 mS/cm (fully hydratedcondition) at 80° C. (FIG. 5) and showed no evident conductivity loss upto 35 days when the membrane was exposed to 1M KOH at 80° C. (FIG. 6).

The current disclosure demonstrates the synthesis of cationicmetallocene-containing vinyl monomers. The monomers include styrenic,cyclooctene, norbornene and cyclic monomers. FIG. 1 illustrates examplesof styrene, cyclooctene, norbornene and cyclic-type cationicmetallocene-containing cationic vinyl monomers. a generalembodiment—vinyl monomers with metallocene functional groups. Variouslinkers (e.g. alkane, alkene, alkyne, aromatic ring, ether, imine, amineor combination of above as the linker) can be applied between themetallocene and vinyl groups. For example, cyclooctene basedcobaltocenium-containing monomer is prepared by click reaction between5-azidocyclooct-1-ene and ethynylcobaltocenium hexafluorophosphate, asshown in FIG. 2. The metallocene cations can also be multi-substitutedby methyl groups FIGS. 1. 5-1 to 5-8. (e.g. dimethyl, tetramethyl,octamethyl cobaltocenium cations). These polymers do not containfunctional groups, e.g. esters and amides, that are easily hydrolyzed,decomposed or degraded under strong basic solution.

The current disclosure provides for the synthesis ofmetallocene-containing cationic polymers by free radical polymerizationas well as other polymerization methods. For example, cyclooctene basedcopolymers can be prepared by ring-opening metathesis polymerization(ROMP), as displayed in FIG. 2. The molecular weight of such copolymersis in the range of 1,000 g/mol to 200,000 g/mol. These polymers can betuned by changing the linkers between metallocene functional groups andvinyl groups, or by altering the nature of the metallocene and/or thenumber of the repeating units. In a preferred, ROMP is the method forpreparing related polymers.

The current disclosure has characterized the conductivity and alkalinestability of the side-chain cobaltocenium-containing copolymers. Forexample, one representative with 50% molar ratio cyclooctene basedcobaltocenium monomer exhibits favorable mechanical (over 100%elongation) and electrical properties, especially at higher temperature,possessing a conductivity over 90 mS/cm at 90° C., as demonstrated inFIG. 4 and FIG. 5. Furthermore, over 95% of initial hydroxideconductivity of hydrated membranes was maintained after soaking in 1MKOH at 80° C. for one month, as shown in FIG. 6.

The method presented in this disclosure offers the following keyfeatures: metallocene-containing functional group can be integrated aspart of vinyl monomer units; the metallocene moiety can becobaltocenium, rhodocenium and/or their derivatives; metallocenecontaining vinyl monomers used for preparation of metallocene-containingcationic polymers by various polymerization methods; styrenichomopolymers, cyclooctene homopolymers, norbornene hompolymers and/or ormetallocene-containing main-chain homopolymers; side-chain andmain-chain cationic metallocene-containing random copolymers; side-chainand main-chain cationic metallocene-containing block copolymers,including linear copolymers, star copolymers, bottle-brush copolymers,etc., as known to those of skill in the art; and improvedelectrochemical properties of such cationic metallocene-containinghomopolymers and various copolymers. Further, the properties of cationicmetallocene-containing polymers can be tuned by changing the monomerstructures (the polymerizable vinyl moiety, the linker or themetallocene moieties), compositions of various copolymers.

FIG. 7 shows mechanical properties of anion-exchange membranes based onmulti-substituted cobaltocenium: normal cobaltocenium (blue),dimethyl-cobaltocenium (orange) and octamethyl cobaltocenium (green).

FIG. 8 shows thermal stability of anion-exchange membranes based onmulti-substituted cobaltocenium: normal cobaltocenium (blue),dimethyl-cobaltocenium (orange) and octamethyl cobaltocenium (green).

In a further embodiment, the current disclosure provides a new class ofrobust AEMs based on cobaltocenium-containing polyelectrolytes that bearpromising hydroxide conductivity as well as mechanical, thermal andchemical stability. These novel membranes exhibited long-term stabilitythat is unprecedented over many reported polymeric systems.Specifically, the current disclosure has developed side-chaincobaltocenium-containing vinyl polymers, which have nonhydrolysablehydrocarbon backbones and extraordinarily stable side chains, yet withhigh modulus and toughness.

Cationic cobaltocenium exhibits good stability toward harsh chemicalenvironments in both strong acidic and basic conditions. There isnegligible change in characteristic UV absorption corresponding tocobaltocenium after it was dissolved in a solution of NaOH (pH 14) orHCl (pH 1.5) for two weeks. Cobaltocenium cations with substituent groupin Cp ring had superior thermal and chemical stability over otherreported cations. Such extraordinary stability of cationic cobaltoceniummotivated led the current disclosure to design ultrastablepolyelectrolytes in both cations and other compositions of frameworksfor AEMs.

To construct the new polymeric systems, one needs to avoid theintegration of functional groups (e.g. ester, amide, anhydride,carbonate) and linkers that are susceptible to degradation under strongbasic conditions for a prolonged period of time. Due to theinstabilities of ester or imide group, most cobaltocenium polymersreported in literature could not be used as stable AEMs directly.

Metal cations have been used for AEMs with prior efforts reporting thesynthesis of AEMs functionalized with bis(terpyridine)-ruthenium(H)complexes. These membranes exhibited good ion conductivity andmechanical properties. However, the decline of conductivity in alkalinecondition over a long term was a concern. Others reported permethylcobaltocenium-containing polysulfone as AEMs that exhibitedextraordinary stability of cations and chemical stability of polymers.However, the ion conductivity was mediocre, partially due to the sterichindrance for the pentamethyl group. Further, these polymers were quitebrittle with elongation at about 10%. Others synthesizedpolybenzimidazole polymers with cobaltocenium cations in the mainchain.These membranes showed improved thermal stability and ion-exchangecapacity, but the mechanical and alkaline stability of these AEMs wereinferior because of the poor stability of linkage groups and the rigidpolymer backbones. Both approaches adopted condensation polymerizationto prepare rigid polymers with relatively limited molecular weight. Inaddition, the synthesis was particularly laborious with low yields.

The current disclosure followed three design principles: 1) eliminateany hydrolysable functionalities; 2) maximize the content of hydrocarbonin the compositions; 3) enable polymers with controlled molecularweight. Specifically, the current disclosure used highly efficient clickchemistry to attach cobaltocenium onto a cyclic vinyl monomer, which wasexecuted for ring-opening metathesis polymerization (ROMP), see J.Zhang, Y. Yan, M. W. Chance, J. Chen, J. Hayat, S. Ma, C. Tang, Angew.Chem. Int. Ed. 2013, 52, 13387-13391; Angew. Chem. 2013, 125,13629-13633; b) J. Wei, L. Ren, C. Tang, Z. Su, Polym. Chem. 2014, 5,6480-6488, which is hereby incorporated by reference. The resultantpolymer has only one triazole group as a linker between cobaltoceniumand the polymer backbone. This powerful synthetic tool allows thepreparation of high molecular weight toward mechanically flexible andtough polymers. In addition, these polymers have a polyethylene-likebackbone to warrant mechanical integrity and chemical stability underharsh basic conditions.

We prepared cobaltocenium-containing cyclooctene that was subject toROMP, as illustrated in Scheme 1, see FIG. 9A. FIG. 9A shows Scheme 1,which shows synthesis of cobaltocenium monomer 2, copolymer 3 andhydrogenated polymer 4 as AEMs. 5-Bromocyclooct-1-ene was derived fromcyclooctadiene, and then converted to 5-azidocyclooct-1-ene (1). FIG. 9Bshows images of transparent and flexible cobaltocenium AEMs. FIG. 9Cshows proposed structures of cobaltocenium containing AEMS in ahydroxide solution. A copper-catalyzed click reaction between 1 andethynylcobaltocenium hexafluorophosphate yielded cobaltoceniumcyclooctene with a triazole as the linker (2), which was subsequentlycarried out for ROMP with cyclooctene as a co-monomer with the aid ofGrubbs II catalyst to get a copolymer 3. The molecular weight was wellcontrolled by adjusting the molar ratio of monomers to catalysts. Inthis study, we chose a polymer with molecular weight at 80000 gmol⁻¹,which is sufficiently high to warrant the formation of flexiblemembranes. The cobaltocenium monomer and cationic copolymers inhexafluorophosphate form were characterized by 1H NMR spectroscopy, seeFIG. 10, which unambiguously confirmed their structures with specificassignments to each proton. FIG. 10 shows ¹H NMR spectra ofcobaltocenium monomer 2 (top) in CDCl₃, copolymer 3 (middle) andhydrogenated polymer 4 (bottom) in dimethyl sulfoxide-d₆.

The presence of unsaturated bonds in the backbone of polymer 3 could bea concern on the long-term stability in alkaline conditions. Thealkaline stability of this copolymer was improved by hydrogenation ofthe backbone. The reduction of double bonds could be confirmed with 1HNMR spectroscopy, see FIG. 10, and disappearance of characteristicabsorption in the range of 1686-1796 cm⁻¹ in FTIR spectrum. Thesehydrogenated copolymers were subsequently carried out ion-exchange fromhexafluorophosphate (PF6—) to chloride (Cl—) and then to hydroxide (OH—)ion, and further fabricated to form anion exchange membranesH-AEM_(x)-OH (5) (x represents the molar fraction of cobaltocenium unitsin a polymer composition).

Cobaltocenium monomer 2 with Cl— anion was evaluated for its alkalinestability by UV/Vis spectroscopy, as cobaltocenium has a characteristicUV absorption at approximately 282 nm. The monomer was dissolved in 1MNaOH at 80° C. and then collected for time-dependent UV/Vis spectra. Tworepresentative absorption peaks appeared at 282 nm and 346 nm,characteristically originated from the cobaltocenium and triazolegroups, respectively. The absorption spectrum obtained after 10 daysperfectly overlapped with the initial one. Both peaks maintained nearly100% of initial intensity even after 10 d, unambiguously indicatingexcellent alkaline stability of cobaltocenium and triazole at elevatedpH and temperatures. Furthermore, the stability of cobaltocenium monomer2 was confirmed by ¹H NMR and mass spectrum after exposure in alkalineconditions. The current noticed that proton-deuterium exchange occurredon protons from both aromatic rings and alkenes in the monomerstructure, while no signals indicated any degraded products. This kindof proton-deuterium exchange is reversible and does not lead to anystructural degradation. All above stability tests suggestedcobaltocenium is a stable cation suitable for AEM applications.

For cobaltocenium-containing polymers, direct anion exchange fromhydrophobic PF₆— to hydrophilic OH— was challenging. The currentdisclosure resolved this problem by first exchange to Cl— in 2 M aqueoussolution of NH₄Cl at 60° C. Then, exchange from Cl— to OH— was carriedout by immersing polymer membranes into 1 M NaOH solution. Completeion-exchange was confirmed by ion exchange capacity (IEC) titration, seeFIG. 11, Table 1: Properties of Cobaltocenium AEMS, and EnergyDispersive X-ray (EDX) characterization. These membranes were yellow incolor, flexible and transparent, see FIG. 9B. All membranes after anionexchange were not soluble in water or other polar aprotic solvents likedimethylacetamide and acetonitrile.

The thermal stability of cobaltocenium-containing polymers was studiedby thermogravimetric analysis (TGA), see FIG. 12 at (a). FIG. 12 showscobaltocenium AEMS: (a) TGA and DTG curves; (b) SAXS profiles; (c)hydroxide conductivity of membranes as function of temperature in fullyhydrated condition; and (3) room temperature conductivity as a functionof immersion time of membranes in 1 M NaOH solution at 80° C. Thedecomposition of hydrogenated H-AEM_(x)-OH membranes in N₂ atmosphereconsisted of two different stages and occurred only well above 300° C.The 5% weight-loss decomposition temperature (T_(d)) of H-AEM40-OH andH-AEM50-OH were 347° C. and 328° C., respectively. In comparison, manyreported AEMs with ammonium or imidazole cations have much lowerdecomposition temperature for their cations (<200° C.). See) D. Guo, A.N. Lai, C. X. Lin, Q. G. Zhang, A. M. Zhu, Q. L. Liu, ACS Appl. Mater.Interfaces 2016, 8, 25279-25288; b) H. Wu, W. Jia, Y. Liu, J. Mater.Sci. 2017, 52, 1704-1716; c) Q. Ge, J. Ran, J. Miao, Z. Yang, T. Xu, ACSAppl. Mater. Interfaces 2015, 7, 28545-28553. The results of the currentdisclosure disclose both cobaltocenium cations and the hydrocarbonbackbone have excellent thermal stability.

Mechanical properties of AEMs are largely dependent on the choice ofpolymer backbones. For well-studied polyp-phenylene oxide),polybenzimidazole, and polystyrene based AEMs, the tensile strain atbreak is usually lower than 80% due to their rigid aromatic backbones.While for flexible poly(4-methyl-1-pentene) based films, the tensilestrength is relatively low. The current disclosure provides sturdy andtough cobaltocenium polymers: flexible and strong. Mechanical propertiesof cobaltocenium AEMs were characterized at 80% RH and under ambienttemperature. Both HAEM₄₀-OH and H-AEM₅₀-OH retained tensile strain over120% attributed to the existence of a highly flexible backbone.

These cobaltocenium membranes were expected to have phase separation dueto immiscibility of the hydrophobic backbone and hydrophilic side chainin the copolymers. Small-angle X-ray scattering (SAXS) was used toinvestigate the morphology of cobaltocenium membranes, see FIG. 12 at(b). Both of the membranes exhibited a broad primary scattering peakaround q*=1.28 and 1.54 nm⁻¹ with a d spacing (2n/q*) of 4.9 and 4.1 nm.The lack of higher orders of scattering peaks implied that the phaseseparation is probably short range correlated, which is reasonable giventhe membranes are not block copolymers. Atomic force microscopy (AFM)images also confirmed the microphase separation between hydrophilic andhydrophobic domains under spin casting conditions. The dark areasrepresent the soft matrix from polyethylene backbone, whereas thebrighter ones represent hard domains from cobaltocenium side chains.Such phase-separated morphology is essential for anion exchangematerials with desirable properties. The hydrophobic backbone providedmechanically flexible and chemically stable scaffolds, whereas thehydrophilic side-chains facilitated ion transport through connectedionic channels.

Even under high cation loading, side-chain cobaltocenium AEMs stillexhibited relatively low water uptake and swelling ratio especially atroom temperature, which can be explained by the hydrophobic nature ofpolyethylene-like backbone. Furthermore, the number of absorbed watermolecules per cobaltocenium cation was calculated to be in the range of11.2 to 11.6, and showed a tendency of slight increase with higher IEC,see FIG. 11. Hence, both cobaltocenium copolymers showed good resistanceto water uptake at various temperature. Such water management propertycould play a critical role in forming ion conducting pathways, see FIG.9C, and ensure the membranes to be dimensionally stable over a longterm. While for many quaternary ammonium or phosphonium polymers, highcation loading for better electrical property much more hydrophilic andabsorbs excess water, which in turn sacrifices the overall stability.The current disclosures results on cobaltocenium AEMs indicated thepossibility of simultaneously making high IEC and having good watermanagement properties.

The hydroxide conductivity of membranes with different loading ofcobaltocenium cation as a function of temperature is shown in FIG.12(c). Both membranes showed steadily increased conductivity at elevatedtemperature likely due to higher water uptake and faster ion migration.The hydroxide conductivity of H-AEM₄₀-OH and H-AEM₅₀-OH reached fairlyhigh values at 90° C.: 72 and 90 mScm⁻¹, respectively. Considering theprocessability and durability of membranes, the current disclosure onlyincorporated up to 50 mol % cobaltocenium in copolymers at this stage.It could be possible to achieve even higher hydroxide conductivity byincreasing the IEC and optimizing the compositions of copolymers.Moreover, the hydroxide conductivity at different temperature generallyfollowed an Arrhenius relationship. The activation energy for iontransport was calculated to be 19.2 to 20.2 kJmol⁻¹.

The alkaline stability at high operating temperature has always been akey concern for AEMs. For polymers prepared through ROMP, theunsaturated double bond in repeating units is considered to not onlylimit the chain flexibility, but also make membranes less stable underharsh basic conditions. The current disclosure employed directhydrogenation to reduce the double bonds in the polymer backbone. Inaddition, polymers with polyethylene-like backbone exhibited improveddurability and ductility. H-AEM₅₀-OH was chosen for the long-termstability test because of its higher ion capacity and hydroxideconductivity. FTIR spectra showed that all chemical structures ofmembranes remained almost unchanged before and after the test. Moreover,initial hydroxide conductivity of hydrated membranes was maintained over95% after soaking in 1 M NaOH at 80° C. for one month, see FIG. 12 at(d), indicating their superior chemical and mechanical stability.

In summary, a new design for anion-exchange membranes was achieved usingmetallo-polyelectrolytes containing cationic cobaltocenium with apolyethylene-like backbone and a non-hydrolysable linker. Thesecopolymers were prepared via ROMP in conjunction with hydrogenation ofpolymer backbones. These membranes exhibited long-term durability withexcellent mechanical toughness and flexibility, great chemical stabilityof cations and the rest of polymer compositions, as well as good ionconductivity. This work demonstrated that side-chain cobaltoceniumpolymers can be used as a candidate for new-generation anion-exchangemembrane materials in fuel cell applications.

The copolymers of the present disclosure exhibit improved mechanicalproperties, alkaline stability and swelling behavior. All of these maybe may be tuned by crosslinking or hydrogenation reactions.

Examples of such cationic metallocene-containing vinyl monomers andtheir polymers are described below.

EXAMPLE 1

This example is to prepare a cyclooctene based cobaltocenium-containingmonomer (see FIG. 2). A typical procedure for the synthesis is describedas follows: 5-azidocyclooct-1-ene and ethynylcobaltoceniumhexafluorophosphate (mono-substituted or multi-substituted) aredissolved in tetrahydrofuran (THF)/H₂O, followed by adding coppersulfate and sodium ascorbate under the protection of nitrogen gas at 0°C. The resulting mixture is stirred at room temperature overnight toyield cyclooctene based cobaltocenium-containing monomer.

EXAMPLE 2

This example is to prepare cyclooctene based cobaltocenium-containingmonomer based copolymers (see FIG. 2). The monomers are polymerized byROMP using Grubbs 2^(nd) catalyst and N,N-dimethylacetamide as solventat 65° C. under nitrogen. The obtained polymers were precipitated inmethanol several times to remove catalysts and other impurities.

EXAMPLE 3

This example is to improve the mechanical and alkaline stability forhomopolymers/copolymers (see FIG. 3 at a). The current disclosuredemonstrated that hydrogenation of backbones is an efficient way toimprove the properties of these metallocene polymers. As seen in FIG. 3,the copolymers could be hydrogenated using excess tosyl hydrazide andN,N-dimethylacetamide as solvent at 100° C. under nitrogen or underhydrogen gas. The resulting polymers are precipitated in methanol threetimes to remove impurities. Similarly, hydrogenation can be applied toother multi-substituted cobaltocenium copolymers to prepare AEMs, likedmccAEM and omccAEM shown in this FIG. 3.

EXAMPLE 4

This example is to improve the mechanical and alkaline stability forhomopolymers/copolymers (see FIG. 3 at b). The current disclosuredemonstrated that by crosslinking of polymer backbones, the propertiesof anion exchange membranes are largely improved. As seen in FIG. 3 atb, the copolymers were dissolved in N,N-dimethylacetamide and purgednitrogen for 15 mins. Dithiol crosslinker and photoinitiator were addedsuccessfully and stirred under UV light overnight. The solution was thendirectly cast on a Teflon film and evaporated the solvent to obtain thefilm for anion exchange.

EXAMPLE 5

This example is to improve the electrochemical properties, thermalstability and alkaline stability for homopolymers/copolymers (see FIGS.7 and 8). The ccAEM, dmccAEM and omccAEM represent anion-exchangemembrane based on cobaltocenium, dimethyl cobaltocenium and octamethylcobaltocenium respectively. The current disclosure demonstrated that byusing multi-substituted cobaltocenium cations, the properties of anionexchange membranes can be tuned. The preparation of multi-substitutedcobaltocenium-based anion-exchange membranes are similar as mentionbefore.

The above examples show that many other polymer systems may be appliedin a similar fashion to obtain controlled properties. The strategydescribed here not only offers the diversity of structures of differentmonomer and polymer systems, but also tailored properties. Theimplementation of this strategy enables cationic metallocene-containingpolymers for anion exchange membranes.

While the present subject matter has been described in detail withrespect to specific exemplary embodiments and methods thereof, it willbe appreciated that those skilled in the art, upon attaining anunderstanding of the foregoing may readily produce alterations to,variations of, and equivalents to such embodiments. Accordingly, thescope of the present disclosure is by way of example rather than by wayof limitation, and the subject disclosure does not preclude inclusion ofsuch modifications, variations and/or additions to the present subjectmatter as would be readily apparent to one of ordinary skill in the artusing the teachings disclosed herein.

What is claimed is:
 1. An anion-exchange membrane comprising: a cationicmetallo-polyectrolyte comprising a polymer backbone with a triazolegroup linker between the polymer backbone and cobaltocenium; and whereinthe anion-exchange membrane has a polyethylene-like framework andalkaline-stable cobaltocenium for ion transport.
 2. The anion-exchangemembrane of claim 1, wherein the membrane is flexible.
 3. Theanion-exchange membrane of claim 1, wherein the polymer backbonecomprises nonhydrolysable hydrocarbon.
 4. The anion-exchange membrane ofclaim 1, wherein the anion-exchange membrane is not soluble in polaraprotic solvents.
 5. The anion-exchange membrane of claim 1, wherein thepolymer backbone is hydrophobic.
 6. The anion-exchange membrane of claim1, further comprising hydrophilic side-chains to facilitate iontransport.
 7. The anion-exchange membrane of claim 6, wherein iontransport is facilitated via connected ionic channels.
 8. The anionexchange membrane of claim 1, wherein the anion exchange membraneexhibits increased conductivity as temperature increases.
 9. A methodfor preparing an anion-exchange membrane comprising: performing acatalyzed click reaction to attach cobaltocenium onto a cyclic vinylmonomer; performing ring opening metathesis polymerization of the cyclicmonomer, wherein the cyclic vinyl monomer comprises cobaltocenium withtriazole linker groups; performing backbone hydrogenation of a resultingcopolymer of the ring opening metathesis polymerization to form ahydrogenated copolymer; and producing a polyethylene-like framework andalkaline-stable cobaltocenium cation for ion transport.
 10. The methodof claim 9, further comprising eliminating hydrolysable functionalitiesfrom the anion exchange membrane.
 11. The method of claim 9, furthercomprising the hydrogenated copolymer subjected to ion-exchange fromhexafluorophosphate (PF₆—) to chloride (CO and then to hydroxide (OH—).12. The method of claim 9, wherein the cyclic vinyl monomer comprisescyclooctene.
 13. The method of claim 9, wherein the anion-exchangemembrane is not soluble in polar aprotic solvents.
 14. The method ofclaim 9, wherein the anion-exchange membrane is flexible.
 15. Ananion-exchange membrane formed via a reaction scheme comprising: